[slider] The "x" value is not preserved when gesture interrupts

[aleclarson]

It would be nice if x in the "Gesture Slider" demo was preserved when its animation is interrupted by another gesture.

Steps

1. Set the friction to ~500

2. Drag the slider left/right and release

3. Press down on the slider before x hits 0

4. Notice how the x animation is not stopped when down becomes true

5. Notice how x jumps to 0 when you drag left/right